The storage and delivery of ultra-high purity (UHP) gases is a critical issue to industry, particularly the electronics industry. To prepare a storage vessel or delivery manifold for ultra-high purity gas service, all the constituents of ambient air must be thoroughly removed from the system. Atmospheric contaminants, such are oxygen, nitrogen, and argon are gaseous and do not adsorb strongly on the metal walls of the vessel or delivery system. These gases are therefore easily removed from the system by purging with an inert gas, evacuating the system, or cycling the system between pressurized inert gas and vacuum.
Atmospheric moisture is different. It readily condenses on metal surfaces in multiple layers. Under normal atmospheric conditions less than 1 molecular layer of oxygen or nitrogen will physically adsorb on a metal surface. Under the same conditions, up to 125 molecular layers of moisture will adsorb on the metal. Moisture also adsorbs to metal surfaces more strongly than does oxygen or nitrogen. The activation energy of desorption for oxygen from a metal surface is about 3-4 kcal/mol. The activation energy of desorption of moisture is typically 15-20 kcal/mol. This large difference in activation energy corresponds to the desorption rate of moisture being about 100,000,000 times slower than the desorption rate of oxygen. This strong adsorption of multiple layers of moisture makes complete removal of moisture from a system a very difficult task. Typically, moisture is removed by purging or evacuation for long periods of time. In some cases it takes several weeks to adequately remove moisture from a delivery system. This is an expensive, time consuming process. Sometimes systems are heated to high temperature to reduce the time required to remove moisture. However heating is not always practical, and it does nothing to prevent re-adsorption of water if the system is again exposed to ambient atmosphere.
In many cases, moisture is the critical contaminant in the gas delivery system. This is especially true when the gas is corrosive. Gases such as hydrogen chloride, hydrogen bromide, fluorine, tungsten hexafluoride, and other halogen containing gases will severely corrode many metals if moisture is present. Corrosion of the storage vessel or delivery manifold can result in introduction of impurities, particles or gas-phase, into the ultra-high purity gas or in extreme cases failure of the system. Component such as valves, regulators, and mass flow controllers are very susceptible to failure due to corrosion and frequently need to be replaced. However, if moisture is rigorously removed, these gases will not corrode commonly used metals such as stainless steel and aluminum. Methods are needed to rapidly remove adsorbed water and passivate the metal surface such that re-adsorption of water is inhibited. Such methods would shorten the time required to completely remove moisture from a system and protect expensive components from failure.
Specifically a method is required which can meet the following needs.
1. Reduce the amount of time it takes to dry down a system to a specified moisture level. PA1 2. Generate a hydrophobic surface that inhibits water from re-adsorbing after the treatment. PA1 3. Enhance point-of-use purity for gases. PA1 4. Improve the corrosion resistance of the materials of construction. PA1 5. Enhance stability of the process gas, especially gas mixtures having a low concentration level of one component. PA1 6. Prevent moisture transients from being dampened.
Previous investigators have developed methods for chemically removing moisture from a metal surface. However, none of these methods have been shown to produce a stable hydrophobic surface. Y-E. Li, J. Rizos, and G. Kasper (U.S. Pat. No. 5,255,445 and Canadian Patent Application number 2,070,145) disclose a method to dry a metal surface to enhance the stability of a gas mixture contacting such surface. Their method is to expose a purged metal surface to a drying agent consisting of one or more gaseous hydrides in low concentration. In their examples, they show that the stability of a low concentration mixture of arsine in argon is improved if the cylinder is first treated with a silane. However, if the metal surface is re-exposed to moisture the beneficial effect of silane treatment is destroyed. This demonstrates that silane treatment does not produce a stable hydrophobic surface.
K. Tatenuma, T. Momose, and H. Ishimaru (J. Vac. Sci. Technol. A, 11, 1719 (1993)) and Japanese Patent number 177299 describe a method to chemically remove moisture using reactive organic halides such as COCl.sub.2 and CH.sub.3 CCl.sub.2 CH.sub.3, at either room or elevated temperature. These compounds react with surface bound moisture to form gaseous by-products which are more easily removed than moisture. Their experiment was to expose a UHV vacuum chamber to a vapor of the moisture-reactive chemical for 10 minutes between 1 and 5 times. The time for the system to pump down to 10.sup.-7 and 10.sup.-8 torr was then measured and compared with the pump-down time of an untreated chamber. Treatment with CH.sub.3 CCl.sub.2 CH.sub.3 was found to dramatically shorten the pump-down time. Treatment with chlorotrimethylsilane was found to have little or no effect on shortening the pump-down time as reported in Table 1 of the article. Experiments to determine if surface treatment was stable to re-exposure to moisture were not performed.
The present invention overcomes the drawbacks in the prior art of preparing piping for ultra high purity gas delivery service by using a class of reagents in a novel process to reduce the amount of time it takes to dry down a system to a specified moisture level, generate a hydrophobic surface that inhibits water from re-adsorbing after the treatment, enhance point-of-use purity for gases, improve the corrosion resistance of the materials of construction, enhance stability of process gas, especially gas mixtures having a low concentration of a component, and prevent moisture transients from being dampened; as set forth in greater detail below.